This invention is related to rolling element bearings, and in particular, to a coating for the cage of such a bearing.
Rolling element bearings comprise an inner ring and an outer ring each provided with a raceway. At least one of the rings has land regions on opposite sides of the respective raceway. A series of rolling elements which are in rolling contact with the raceways are maintained in a spaced-apart relationship by a cage. The cage engages the land regions of the ring.
It is a known practice to apply solid lubricant coatings such as silver to the cages in rolling element bearings for some applications. The silver coatings on the cages have two functions: (1) to form transfer films on the rolling elements and raceways and (2) to provide a barrier to adhesive wear between the machined steel cages and the land regions of the rings. A prevalent problem with this approach is that debris still present from the manufacturing process of the system, ingested into the system during operation, or evolved in the system through normal operation becomes trapped in the silver coating. Debris trapped in the silver coating is especially harmful when it is located on the regions of the cage that contact the land regions on the rings. The trapped debris behaves as grinding media and abrasively, or adhesively (depending upon the debris type) wears material from the land surfaces. When a critical amount of material has been removed from the land surface, the bearing fails catastrophically.
For example, EP 531082 discloses a bearing having rolling elements spaced by a cage which rides on the lands provided on one of the rings next to the raceway thereof. Such bearing has the advantage that the cage is guided by the ring in question, which improves the dynamic behavior of the ring and reduces whirl instability. According to EP 531082, a hard coating is applied to each land of the ring. According to U.S. Pat. No. 4,997,295, WS2 and/or MoS2 coatings are applied to the rolling elements and the inner and outer rings of rolling element bearings. According to U.S. Pat. No. 5,067,826, synthetic diamond coatings are applied to the rolling elements of bearings. Synthetic diamond is not useful as a coating for rolling element bearings because of its high deposition temperature, its low fracture toughness, and its inability to elastically accommodate contact stresses. U.S. Pat. No. 5,112,146 describes the use of a functionally, graded material system for the raceways of bearing rings. U.S. Pat. No. 5,593,234 describes the use of polycrystalline super-lattice coatings applied to the raceways of bearing rings. U.S. Pat. No. 6,471,410 describes the use of coatings on the land regions of the ring but not the cage thereof.